coils on cb antenna
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n0nhp
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To resonate at a certain frequency and match the output impedance of the transmitter (In this case around 27MHz and 50 Ohms) the antenna needs to be a certain length. The coils you see allow the physical length to be reduced but the electrical length to be maintained.
The physical length and diameter are up to the designer of the antenna.
If the designer wants the antenna to be able to handle lots of power the physical size must be adequate to handle the current.
In other words, if you are running a legal transmitter, the coil doesn't need to be the size of a Foster's pony keg.
Rule of thumb, the longer the receive area of the antenna, the more signal you will receive. A 108" stainless steel whip will handle all the power you can throw at it and provide the best reception area for the signals you are trying to receive. It is awfully noisy when you pull into the carport though.
If you are serious about building your own antennas may I suggest ARRL Antenna Book as a great place to start. It will cover the theory and give plans and suggestions that are tried and true.
Bruce
The physical length and diameter are up to the designer of the antenna.
If the designer wants the antenna to be able to handle lots of power the physical size must be adequate to handle the current.
In other words, if you are running a legal transmitter, the coil doesn't need to be the size of a Foster's pony keg.
Rule of thumb, the longer the receive area of the antenna, the more signal you will receive. A 108" stainless steel whip will handle all the power you can throw at it and provide the best reception area for the signals you are trying to receive. It is awfully noisy when you pull into the carport though.
If you are serious about building your own antennas may I suggest ARRL Antenna Book as a great place to start. It will cover the theory and give plans and suggestions that are tried and true.
Bruce
IF the whip has the proper groundplane. And the groundplne IS the other half of the antenna, does not work whithout it.
Thanks for the replies. I'm looking into that book you suggested, Bruce
I assume that on a big rig the ground plane would become the chassis of the truck.
I've also seen some of these drivers slanting the antenna forward almost 45 deg. Isn't this action counter productive to receiving signal?
I assume that on a big rig the ground plane would become the chassis of the truck.
I've also seen some of these drivers slanting the antenna forward almost 45 deg. Isn't this action counter productive to receiving signal?
AC9BX
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A big truck is perhaps the worst vehicle to attempt to install an antenna on. It's a miserable structure causing terrible imbalanced signal patterns. All the metal off to one side. The best place would be right in the middle of the cab roof. But most don't want to nor would they be permitted if they aren't owners to go drilling holes in their truck. So you end up with mirror mounts which by default work poorly. Those crazy angles some try may or may not help. I doubt it does much good at all.
n0nhp
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Most modern big rigs have fiberglass cabs. This is a problem as they do not provide the ground plane that cmdrwill spoke of causing the shield of the cable and mirror bracket. to try to become the other half of the antenna. Almost every trucker using 27MHz CB will tell you that you will need to turn off all other audio equipment in the truck when using the transmitter due to RF Interference caused by stray RF in the cab.
With commercial VHF / UHF equipment I used to place a stainless steel groundplane plate with the antenna mounted in the center of the cab. Fun days drilling holes in half million dollar rigs with less than a hundred miles on the clock.
I have not looked at what the truck stops are selling for CB antennas now days but in my opinion the best result might be from a center fed loaded dipole mounted vertically on the mirror, and no I don't know of any that are manufactured like that.
The forward lean on the mirror mounts are as far as I can tell, only for the rakish look. The physics of the antenna would tell you that they are putting the most of their signal into the road bed in the 100 yards just in front of them, a off kilter signal to their sides and pointing at the sky behind them.
The techs at the Boise fire center did some tests (on VHF but the physics stay the same) when firefighters started using chest harnesses for their radios. Those harnesses effectively do the same thing that tilting the antennas on the trucks do. The signal was diminished by a minimum of 2.7 to 3 dB to a receiver that had a vertical antenna. 3dB is half power. If the antenna was tilted the same amount but opposite the attenuation was closer to 6dB indicating that only 1/4 of the rf was being induced into the receiving antenna. I always thought that being able to communicate to other rigs ahead was the reason for the radios in the first place so the drivers had an idea what road conditions were in front of them.
I guess like many other things in this world it comes down to form over function.
DO NOT take my or any other "expert's" word for it though, one of the most fun things about this hobby is that radio signals will do the most unexpected things using the most unexpected materials. Experiment, sit down and analyze, rebuild, experiment again and keep learning.
Bruce
With commercial VHF / UHF equipment I used to place a stainless steel groundplane plate with the antenna mounted in the center of the cab. Fun days drilling holes in half million dollar rigs with less than a hundred miles on the clock.
I have not looked at what the truck stops are selling for CB antennas now days but in my opinion the best result might be from a center fed loaded dipole mounted vertically on the mirror, and no I don't know of any that are manufactured like that.
The forward lean on the mirror mounts are as far as I can tell, only for the rakish look. The physics of the antenna would tell you that they are putting the most of their signal into the road bed in the 100 yards just in front of them, a off kilter signal to their sides and pointing at the sky behind them.
The techs at the Boise fire center did some tests (on VHF but the physics stay the same) when firefighters started using chest harnesses for their radios. Those harnesses effectively do the same thing that tilting the antennas on the trucks do. The signal was diminished by a minimum of 2.7 to 3 dB to a receiver that had a vertical antenna. 3dB is half power. If the antenna was tilted the same amount but opposite the attenuation was closer to 6dB indicating that only 1/4 of the rf was being induced into the receiving antenna. I always thought that being able to communicate to other rigs ahead was the reason for the radios in the first place so the drivers had an idea what road conditions were in front of them.
I guess like many other things in this world it comes down to form over function.
DO NOT take my or any other "expert's" word for it though, one of the most fun things about this hobby is that radio signals will do the most unexpected things using the most unexpected materials. Experiment, sit down and analyze, rebuild, experiment again and keep learning.
Bruce
And remember, almost any antenna in the air works better than the one you never install because some "expert" tells you it won't work!
Thanks for the input. I have tried to tell some of these guys that their antennas are not getting out right leaning them forward, but they don't listen. I guess they think it looks cool.
Now, if i wanted to run a little more "juice" through the wires to the antenna I'm thinking I might need to see if the factory coaxial could handle it so as not to start a fire in the truck. Any idea how to check on that info. or is there a chart for size of cable to power capability?
Now, if i wanted to run a little more "juice" through the wires to the antenna I'm thinking I might need to see if the factory coaxial could handle it so as not to start a fire in the truck. Any idea how to check on that info. or is there a chart for size of cable to power capability?
The reason some trucks have the tilted forward antenna is tunning . When they are verticle they are running parralel with the a post 4 inches away. Tilting them reduces the interactance and the swr. This is mainly on the frieghtliner cascadia that mounts the antenna to the mirror at the bottem of the window.
When I drove I had a browning antenna. I loved it. Picked it up at my local radio shop that has now become extinct. It is the best antenna I bought for my cb. Handled all I can toss at it.
The reason I had my antenna tilted forward was at 65 mph the whip tip would be almost vertical even of it was bowed back. If I had it up the antenna would be so to say bent backwards while going down the road.
My local tractor trailer I drove I kept the antenna actually tilted backwards since a lot of my deliveries were on residential streets (mason brick and wallstone manufacturer) I did not want the antenna to spear the trees and get caught in the branches. I did not care for distance but my radio was pretty good with the selectivity.
The reason I had my antenna tilted forward was at 65 mph the whip tip would be almost vertical even of it was bowed back. If I had it up the antenna would be so to say bent backwards while going down the road.
My local tractor trailer I drove I kept the antenna actually tilted backwards since a lot of my deliveries were on residential streets (mason brick and wallstone manufacturer) I did not want the antenna to spear the trees and get caught in the branches. I did not care for distance but my radio was pretty good with the selectivity.
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